Core Graphics rendering surface #5

Set up a Core Graphics bitmap rendering surface in the AppKit window.

Required FFI:

• CGContext, CGBitmapContext (CGBitmapContextCreate, CGBitmapContextGetData)
• CGColorSpace (CGColorSpaceCreateDeviceRGB)
• CGImage (CGBitmapContextCreateImage)

Implementation:

• Custom NSView subclass overriding drawRect:
• Create a CGBitmapContext backed by a Rust-owned pixel buffer
• Draw the CGImage into the view's context in drawRect:
• Helper to fill the buffer (draw a colored rectangle as proof of life)

Acceptance criteria:

• Window shows a colored rectangle rendered via CG bitmap
• Pixel buffer is owned by Rust, CG just displays it

Depends on: AppKit window creation